Negative corona discharge system using alternating electric fields across the air gap



March 18, 1969 c. F. GALLO 3,433,948

NEGATIVE CORONA DISCHARGE SYSTEM USING ALTERNATING ELECTRIC FIELDS ACROSS THE AIR GAP Filed April 6, 1967 FIG. 2A FIG. 2B

A.C. FIG. 4 U

INVENTOR. CHARLES F. GALLO United States Patent NEGATIVE CORONA DISCHARGE SYSTEM USING ALTERNATING ELECTRIC FIELDS ACROSS THE AIR GAP Charles F. Gallo, Penfield, N.Y., assignor to Xerox Corporation, Rochester, N.Y., a corporation of New York Filed Apr. 6, 1967, Ser. No. 628,995 US. Cl. 25049.5 7 Claims Int. Cl. H013 37/26 ABSTRACT OF THE DISCLOSURE This invention relates to a negative corona discharge system for the uniform charging of a charge-retaining surface. A high voltage direct current potential source is applied across the air gap between the corona discharge electrode and the surface to be charged in the conventional manner while alternating electric and/ or magnetic fields are applied across the air gap. The presence of the field influences the path of the electrons and negative ions as they migrate from the electrode to the surface being charged in such a manner that they are deposited more uniformly on the surface. The field is generated either by shorting an alternating current source across the length of the corona discharge electrode or by using an external field generator.

Background of the invention A corona wire is commonly used to place positive and negative charges on charge-retentive surfaces. For instance, the corona technique has been used to charge photosensitive surfaces of xerographic plates prior to exposing the plates to a pattern of light and developing the latent image. When a positive corona wire is used in the charging process, the resultant charge on the photosensitive surface is relatively uniform. However, when a negative corona wire is used to charge the plate, particularly if the charging time is of relatively short duration, the photosensitive surface receives a charge which varies in density from point to point. This density variance is an undesirable condition on a xerographic plate since it does show up on the reproduced copies.

Several variations have been made on the negative corona apparatus in an attempt to make the resulting charge density on the xerographic plate more uniform. One modification which aids in the deposition of a uniform negative charge on a surface is taught in US. 2,856,533 to Rosenthal. The discharge electrode is a corona wire formed into an endless loop and supported by two movable pulleys. The pulleys move the corona wire in such a manner that any particular point on the wire rotates around the entire loop continuously as charging proceeds. A corona wire when activated by a negative potential normally emits electrons sporadically along its length which tends to deposit a non-uniform charge on the surface being charged. The effect of the rotating corona wire is to average out the emission points on the wire thereby increasing uniformity. Although this system does create a more uniform charge than was previously possible, it is only an averaging method and some non-uniformity still results. In addition, a supporting mechanism is necessary to rotate the corona wire which requires a good ideal of additional space in an otherwise compact charging device.

Another variation in negative corona apparatus consists of placing a high-frequency alternating current in series with high voltage direct current across the corona wire. Likewise, this arrangement generates a more uniform charge density on a surface, but is not economically practical since costly equipment must be used to develop the 3,433,948. Patented Mar. 18, 1969 high-frequency required for good uniformity ,and to introduce it into the high voltage direct current line.

Without utilizing the modifications mentioned above, a photosensitive surface can be charged somewhat more uniformly by increasing the voltage of the direct current on the negative corona wire. As the voltage is increased beyond the point where corona discharge first begins to take place, electron emission points will move closer together on the corona wire. However, in the range of practical potentials, the corona never becomes sufficiently uniform. Higher voltages are undesirable since the accompanying increased ozone production is toxic and acts as a strong oxidizer. Some improvement in the uniformity of negative corona discharge also is effected by keeping.

the corona wire extremely clean, smooth and polished or, alternatively, by pitting the corona wire by some process such as sandblasting.

Another method used in improving uniformity of the charge is by separating the surface to be charged from the corona wire an adequate distance so that the groups of electrons emitted have adequate distance to spread out and overlap before being deposited on the surface. The resulting charge is always more uniform with the separation, but there is an inordinate loss of efiiciency in the corona wire. It is also possible to achieve uniform charging of a surface by thermionic emission. The drawbacks in this method are that the corona wire must be heated to a sufficiently high temperature to assure good emission and thermionic emission is best conducted in a vacuum because the materials which are good thermionic emitters also react easily with impurities in the atmosphere.

The present invention is intended to improve upon the prior art of negative corona charging. Accordingly, it is an object of this invention to provide a novel negative corona apparatus which overcomes the deficiencies of the prior art devices as described above.

It is a further object of this invention to provide an improved negative corona charging apparatus which deposits a charge of uniform density on a charge-retentive surface.

A still further object of this invention is to provide an improved corona charging apparatus which produces an increased number of emission points on the corona wire.

A still further object of this invention is to provide an improved negative corona charging apparatus which produces an increased number of emission points on the corona wire by the introduction of low-frequency alternating current to the corona wire.

A still further object of this invention is to provide an improved negative corona charging apparatus which deposits a uniform negative charge on a photosensitive surface.

Summary The objects of this invention are accomplished, generally speaking, by applying a high voltage direct current potential source across the air gap between a corona discharge electrode and a charge-retaining surface while an alternating electric and/ or magnetic field exists in the air gap. The field may be applied either by shorting a source of alternating current across the length of the corona or by using an external field generator. The introduction of the field in the air gap acts on the ions so as to alter the route they take to the charge-retaining surface thereby causing more uniform charging. When the field is a mag netic field generated by a source of alternating current shorted across the corona electrode, uniform charging is further enhanced since more emission points result along the electrode than when a direct current is applied to the electrode alone.

Brief description of the drawings For a better understanding of the invention as well as 3 other objects and further features thereof, reference is had to the following detailed description of the invention to be used in connection with the accompanying drawings, wherein:

FIGURE 1 is a schematic diagram of the electrical circuit of the invention;

FIGURES 2A and 2B are schematic diagrams showing the effect of a magnetic field set up by the alternating current upon emitted electrons and negative ions;

FIGURE '3 is a schematic diagram of an alternative apparatus which alters the trajectory of the electrons and negative ions to assure uniform charge density on the surface being charged; and

FIGURE 4 is a schematic diagram of a preferred embodiment of the apparatus to best assure the deposition of a uniform charge.

Description preferred embodiments The surface to be charged can be any type which has the ability to take a negative charge and hold it for some duration of time. For instance, a xerographic plate 11, as shown in FIGURE 1, consisting of photosensitive layer 12, the surface to be charged, overlaying a conductive substrate 13 is used for the purposes of this explanation. The photosensitive layer of the plate consists of any suitable material such as vitreous selenium while the conductive backing should be a material having sufficient electrical conductivity to release the electrical charge on layer 12 upon exposure of the recording surface. However, the exact construction of the element being charged need not be limited to the foregoing description. The only limitation on the construction of the element being charged is that after charging is completed there be a potential difference between the negatively charged side of the element and the opposite side. Hence, if the ele ment being charged were made of a unitary material rather than a two layer material as described above, the side of the element opposite that being negatively charged should be grounded or, preferably, kept at a positive potential as charging herein described is taking place.

Plate 11 is connected in series with a source of direct current 14 and corona wire 17, the plate backing being connected to the positive terminal and the corona wire being connected to the negative terminal. A source of alternating current 16 is connected directly across the length of wire 17. Corona wire 17 extends across plate 11 and the alternating current is connected across the corona wire so that the entire length of plate 11 can benefit from direct corona emission. There may also be a means (not shown) to move the corona wire with respect to the plate in order that the complete surface of the plate may be charged.

The circuit illustrated in FIGURE 1 is completed between the corona wire and plate by electron and negative ion migration from corona wire 17 to the surface of plate 11. As a result of the ionization process naturally taking place in the atmosphere, a small number of free electrons and ions are normally present in the air. By putting an adequate negative potential on the corona wire, the free positive ions surrounding the corona wire move toward the wire with sufiicient velocity to ionize some of the neutral gas molecules they strike. In this manner additional ions and free electrons are produced. The newly created positive ions are themselves accelerated towards the corona wire and collide with other neutral gas molecules producing still more ions and electrons. The process multiplies and the corona wire soon becomes surrounded by a sheath of ions and electrons. The free electrons and negative ions are repelled by the negative potential on the corona wire and migrate to the insulating surface of plate 11 thereby giving the surface a negative electrostatic charge.

While the above process is occurring, it has been observed that as the negative voltage on the corona wire is increased, corona discharge commences and there appears discontinuous and approximately periodic electron emission points along the corona wire seen as reddish tufts of glowing gas. If the voltage of the direct current were increased further in the absence of the alternating current generated by source 16, the glowing points on the wire would move progressively closer together and their number would increase, but, in the range of practical potentials, the corona would never become adequately uniform. By imposing an alternating current on corona wire 17, it is possible to obtain a much more uniform charge density on the surface of plate 12 by corona discharge. The presence of the alternating current across the corona wire causes the number of emission points, evidenced principally by the red tufts of glowing gas mentioned above, to increase vastly per unit length of corona wire as compared to prior art corona generating systems. The reason for the increase in distinct emission points is now known, nonetheless, it is fairly certain that introducing alternating current across the corona in this manner does not cause thermionic emission; i.e., causing emission of electrons by heating the corona wire. Thermionic emission seems an unreasonable explanation of the phenomenon for at least two reasons. During thermionic emission, the distinct emission points along a corona wire tend to broaden or smear as the wires temperature increases giving rise to a continuously emitting corona. Rather than having such a smearing effect, the emission points of the present device simply increase in number while remaining distinct. A second reason that thermionic emission is unlikely in the present device is that the temperature of the corona wire remains well below the temperature necessary to commence adequate emission for good charging. Regardless of the explanation, the increased number of emission points on the corona cause a more uniform charge density across the surface being charged since the electrons emitted tend to be more evenly dispersed. The alternating current can be any frequency approximately 5 cycles per second and above, however, the device is more efficient as the frequency is increased. Nevertheless, the number of emission points increase sufficiently to cause uniform charging even at relatively low frequencies; e.g., at '60 cycles per second, although the higher the frequency, the more uniform will the resulting charge be.

By introducing alternating current across the corona wire, a magnetic field will result around the wire and the direction of this field will depend on the direction of the current flowing through the corona wire at any given instant. Alternating current is of such a nature that across a given conductor the polarity is periodically reversed causing the electrons to first flow in one direction and then in the opposite direction. In FIGURE 2A, the current in the wire is flowing to the left and the direction of the magnetic field surrounding the wire is in the direction as shown by the curved arrow around the corona wire. As free electrons are emitted from the corona wire and as the ionization process continues to create free electrons and negative ions, the free electrons and negative ions are subjected to two distinct forces. The first force results from the aversion that the free, negatively charged electron or ion has for the negatively charged corona wire. This force, represented by Cy, will cause the electron or ion to proceed away from the corona wire. The second force on the free electrons and negative ions is that exerted by the magnetic field surrounding the corona wire and is represented by Cx. In FIGURE 2A, electron 19 is emitted while the magnetic field surrounding the corona wire is in the direction shown. Under these circumstances the direction of force Cx is to the right, substantially in the opposite direction as the current in the corona wire at that instant. The combination of the two forces described a bove cause anemitted electron to take an indirect, more devious route to the surface being charged. Similarly, a newly created negative ion located near the corona wire is acted upon by the field surrounding the corona wire causing it to be influenced by a force Cx and proceed to the plate by a relatively indirect path.

Referring now to FIGURE 2B, at the instant shown corona wire 18 current is .flowing toward the right and the direction of the magnetic field surrounding the wire is in the direction as represented by the curved arrow around the wire. Under these circumstances, Cy is still in a direction perpendicular to the surface of emission, but force Cx is now to the left, again substantially in the opposite direction of the flow of current in the wire. The actual trajectory of electron or ion 19 (dotted line) as shown in FIGURE 2A is dilferent from that as shown in FIGURE 2B because of the reversed direction of force Cx. However, in each case the electron takes an indirect, devious route to the surface being charged. These two illustrations are meant to be only the two extremes of a continuously varying condition. In the time between the conditions represented in FIGURES 2A and 2B, the magnetic field around the corona wire must lessen in strength to zero and then reverse direction and again rise in strength because the fiow of electrons through the corona wire changes direction periodically. Hence, the Cx force exerted on emitted electrons and negative ions varies continuously in strength and direction. The presence of the alternating current, therefore, has the tendency to randomly disperse the electrons and negative ions in a plane along the length of the corona wire, a condition which results in depositing a more uniform charge on the surface being charged.

FIGURE 3 shows an alternative device for generating an electric field used to disperse the electrons and negative ions so that a more uniform change is achieved on plate 23. Corona wire 27 is connected in series with direct current source 22 in such a Way that the wire is charged negatively. The positive terminal of the source of direct current is connected to the conductive substrate of plate 23. Alternating current source 21 is connected to field generators 26, but not to the corona wire. The field generators impose an alternating electric field across the air gap between the surface being charged and the corona wire which acts on the electrons andnegative ions; that is, the electrons and ions take a more devious :path to the surface being charged. The major difference between the fields is that the magnetic field generated around the corona wire as a result of putting alternating current through the corona wire is concentric to the cross-section of the corona wire while field generators 26 create an electric field which is parallel to the wire. Otherwise, the effect of the field upon the electrons and negative ions is similar. Although the field generator shown in FIGURE 3 causes the electrons to be more evenly dispersed after they are emitted from the corona wire, more distinct emission points per unit length of corona wire will not occur as discussed in conjunction with FIGURE 1 because the alternating current is not actually carried through the corona wire.

In order to take full advantage of the greatest number of emission points possible on the corona wire and to achieve the greatest dispersion of electrons and negative ions between the time they are emitted from the corona wire or ionized and are deposited on the surface being charged, a device such as that shown in FIGURE 4 is desirable. The negative terminal of direct current source 33 is placed in series with corona wire 34 while the positive terminal is in series with the conductive backing material of plate 31. Alternating current source 28 is connected across corona wire 34. Alternating current source 29 is connected to field generators 32 which imlpose an electric field in the gap between the corona wire and the surface being charged. In this device more uniform charging initially takes place because there are more emission points on the corona wire. In addition, as electrons are emitted from the corona wire and negative ions are formed, they are subjected to the two fields, both forcing the electron and negative ions in a direction substantially parallel to the length of the corona wire. This condition assures a high dispersion or overlapping of free electrons andv negative ions which, in turn, aids uniformity of the charge resulting on the plate.

In addition to the devices outlined above, many other modifications and/or additions to this apparatus will be readily apparent to those skilled in the art upon reading this disclosure and these are intended to be encompassed within the spirit of the invention.

What is claimed is:

1. A device for depositing a negative charge on an insulating surface comprising:

(a) a direct current source having its positive terminal connected to the insulator portion of said insulating surface,

(b) an elongated corona discharge electrode spaced from said surface and connected to the negative terminal of said direct current source,

(c) an intense alternating current source, and

(d) a field generator connected to said alternating current source positioned so that at least one component of the alteranting field generated by said field generator is perpendicular to a line ta-ken between said corona electrode and said insulating surface.

2. A device according to claim 1 in which said field generator is an electric field generator connected to said alternating current source.

3. A device according to claim 1 in which said field generator is said alternating current source shorted across the length of said discharge electrode whereby the number of electron emission points along said electrode is great thereby resulting in the deposit of a uniform negative electrostatic charge on said insulating surface.

4. A device according to claim 3 further including a second field generator comprising at least one alternating current electrode.

5. A device according to claim 1 in which said alternating current source has a frequency of at least about 5 cycles per second or higher.

6. A device according to claim 5 in which said alternating current source has a frequency of approximately 30 to 150 cycles per second whereby uniform charging is achieved economically.

7. A device according to claim 1 wherein said insulating surface is a photoconductive insulating layer of a xerographic plate, and wherein it is deposited on a conductive substrate connected to the positive terminal of said direct current source.

References Cited UNITED STATES PATENTS 2,885,556 5/1959 Gundlach -1.7 3,013,154 12/1961 Trump 250-495 3,109,931 11/1963 Knowlton 250-495 RALPH G. NILSON, Primary Examiner.

S. C. SHEAR, Assistant Examiner.

US. Cl. X.R. 

